In the production of cold-rolled strip steel, an important process is pickling. Pickling is a chemical process that chemically removes the iron oxide scale from the surface of metal. Iron oxide scales on the strip surface (Fe2O3, Fe3O4, FeO) are alkaline oxides insoluble in water. When they are immersed in acid, these alkaline oxides react with the acid chemically. The oxide scales on the surface of carbon steel or low alloy iron have the property of being loose, porous and with cracks. The cracks will be further enlarged if these oxide scales are straightened, withdrawn-straightened, and transmitted together along the strip steel. Therefore, the acid solution also reacts with the iron matrix of the large strip steel when reacting with the oxide scales. The purpose of pickling is to remove the oxide layer on the surface of the strip steel. Conventional pickling usually has the problem of having lag in acid concentration control, which makes it unable to achieve stable control of hydrochloric acid concentration. The hydrochloric acid concentration fluctuates greatly in the actual production process, resulting in unstable cleaning quality of strip steel, and pretty high usage of acid. This is a waste of hydrochloric acid. Currently, most of the existing pickling sets in the world utilize manual control of acid concentration. It is mainly because the accuracy of real-time measurement of acid concentration is difficult to be ensured, since there are no well-developed measurement and control schemes. Generally, the manual control of acid concentration is implemented such that sampling is carried out manually at regular time, the acid concentration of the sample is analyzed with titration, and the acid is compensated based on actual experiences. The biggest disadvantage of this method is the lag in control. Because the changing of actual acid concentration closely relates to many factors, such as the operating speed of the set, the width of the strip steel, oxidation level of the strip surface and the kind of the steel, etc., manual control method will inevitably cause great fluctuations of acid concentration, resulting in unstable cleaning quality of the strip steel.
Chinese Patent Publication No. CN1462321 discloses a continuous pickling method and a continuous pickling device. The continuous pickling method is that, in one hand, providing acid solution to more than two pickling tanks among several pickling tanks constituting the continuous pickling device, respectively; in the other hand, calculating the total supply amount of the acid with the thickness of oxide scale on the strip steel, the width of the said strip steel, and the conveying speed of the strip when pickling the conveyed strip, and obtaining the distribution proportion of the respective supply amount of the acid to the said more than two pickling tanks with the pickling mode of the said strip and the conveying speed of the strip steel. The pickling line control device predicts the consumed amount of acid stored in the third tank and the final tank of the continuous pickling device during pickling to determine the supply amount of the pickling solution to the third tank and the final tank, respectively, so that the acid is supplied to the acid supply system. The acid concentration measuring device continuously measures the acid concentration of the pickling solutions stored respectively in the third tank and the final tank. The acid supply system supplies the acid to the third tank and the final tank according to the acid concentration values measured continuously, so that the acid concentrations stored in the third tank and the final tank match with respective target value, respectively. This makes the limited acid concentration discharged from the final tank increase to 12%, while makes the acid concentrations of the pickling solutions stored in other pickling tanks increase.
Chinese Patent Publication No. CN1280633 discloses a continuous acid concentration measuring device, wherein it consists a body device that can make the acid pickling solution flow through the channel continuously in one direction, a densitometer, a thermometer and a conductivity meter provided in the body device and actually used to measure the acid pickling solution continuously flowing through the channel, and a calculator used to calculate the concentration of the acid pickling solution according to these measured results. Using this type of acid concentration continuous measuring device can feedback and control the hydrochloric acid concentration in the final acid pickling tank among several acid pickling tanks constituting the continuous acid pickling equipment.
In the disclosed literature mentioned above, it relates to an acid concentration measuring device. The adopted measuring method relates to the temperature measuring, density measuring and conductivity measuring, and to reckoning the acid concentration with the temperature, density, and conductivity. The single-variable acid concentration control method is used. The overall measurement costs a pretty long time, about 1 h. It is an intermittent measuring method. Due to the lag of the acid concentration control, it is impossible to achieve the stable control on the concentration of hydrochloric acid. In the practical production process, the concentration of hydrochloric acid has a large fluctuation, resulting in the instability of the strip cleaning quality. In addition, the consumption of the acid is too large, resulting in a waste of hydrochloric acid.